A soft, warm grey shade would look lovely, and can be as dark or light as you want. A warmer shade will work with the brown stained wood floor, and the grey will complement the black granite. You could have an undertone of green, blue or tan if you didn't want a pure grey. And it would look nice with any silver metal hardware, either polished or satin/brushed, so you could choose pewter, chrome or nickel. It would still work with white trim too.

If you love the marble tile, go for it! You can always buy a stainless or white range hood backsplash like this http://www.wayfair.com/Air-King-Range-Hood-Back-Splash-SP2430-AXK1174.html to slip in place when you are cooking something really messy. I have a travertine backsplash and use a stainless sheet when frying. It works great and no worries about stains ruining the stone.

I think wallpaper is not recommended coz the water splashing might spoil its condition. Rather I suggest you to go for a decorative tile panelling up till the width of the loft and placing a backlit round mirror to get a touch of contemporary style with black/white countertop and white and oak vanity.

I have used both with good results. The main difference between Quartz and Granite are 1) price, 2) colour range. Granite generally works out cheaper, because you can buy the exact quantity you require. Quartz comes in large slabs of specific sizes- standard size being roughly 10'-0" x 4'6", so there tends to be more wastage if you don't plan the cutting well. But Quartz is pretty and comes in a wide range of colours and can be re-buffed at a later date. Quartz also comes in various thicknesses- the most commonly used being .75" to about 1.25". And especially when you use dark wood cabinets in a kitchen or bathroom, beige and white counter tops work very well. Some people go even with coloured counter tops like blue, orange, yellow. Natural stone in light colours tend to be porous and stain easily, so I prefer to go with pure black which is safe.